Gemara

A voice from Heaven: "Zayis Ra'anan... Kara Hash-m Shmech" - just like the greatness of an olive (tree) is in the end, also the greatness of Yisrael is in the end. (They will repent in the days of Mashi'ach.)

99b (Reish Lakish): One who forgets something that he learned transgresses a Lav - "Hishamer Lecha u'Shmor Nafshecha Me'od Pen Tishkach Es ha'Devarim..."

Horayos 13b (Beraisa): Five things make one forget his learning - eating from what a mouse ate from, eating the heart of an animal, eating olives frequently, drinking water left over from washing...

Five things restore one's learning - eating bread baked on coals (and all the more so, the coals themselves), eating a scrambled egg without salt, frequent consumption of olive oil; frequently drinking wine and smelling spices, and drinking water left over from kneading a dough;

Some add sticking one's finger in salt and using that finger to eat.

(R. Yochanan): Just like eating olives makes one forget the Torah he knew for 70 years, olive oil restores the Torah he knew for 70 years.

Ten things cause difficulty in learning - ... eating bread that was not fully baked; eating the froth that accumulates on the spoon used to stir cooking meat; drinking from a stream that passes through a cemetery...

Rishonim

Shitah Mekubetzes (99b): (The text is) a Chacham who forgets transgresses.

Sefer Chasidim (1008): Mice ate from Ploni's bread. He asked a Chacham if he may eat it, for he is hungry, but he is commanded not to eat what will cause him to forget his learning. The Chacham answered 'you are liable only if you remove Divrei Torah from your heart. I see that the entire day you are Batel from Torah, and accompany Amei ha'Aretz to hear idle words. It would be better if you did not avoid things that cause forgetting, so you would forget the idle words you engage in'!

Sefer ha'Zikaron ((of ha'Gaon R. C. Kanievsky Shlita, in Si'ach ha'Sadeh) part 2, introduction): We learn from this that there is not even an Isur mid'Rabanan unless one removes Divrei Torah from his heart. All the Poskim did not bring that cause forgetting. This implies that they are not forbidden. Why didn't they explain what is called 'removing from one's heart'? We must say that since they do not surely cause him to forget, rather, they merely weaken memory, and he eats them with intent for his benefit, (this is not forbidden). However, if one is Batel and does not review his learning, surely he will forget, this is considered removing it from his heart. Surely it is proper to refrain from things that cause to forget. This is why Chachamim listed them!

Maharil (Chadashos 45): The Semak holds that women must learn the Mitzvos that apply to them, and review them in order that they will fulfill them properly.

Shmiras ha'Guf veha'Nefesh (p.99): Since she must review lest she forget, presumably she should avoid things that cause forgetting.

Sefer ha'Zikaron (2, sof introduction): Surely women need not be careful about things that cause forgetting, since they are not commanded about Talmud Torah. Even an Am ha'Aretz need not be concerned (according to the text of Shitah Mekubetzes)! However, we should be concerned for a minor, even before he learned, for things that cause forgetting create a nature, and this can harm the child afterwards.

Shmiras ha'Guf veha'Nefesh (p.98-99): Mishmeres Shalom (72, Sifsei Da'as 2:1) says that even pregnant women must be careful about things that cause forgetting. If so, all the more so a minor must be careful! Pregnant women and males, especially Bnei Torah, should not eat the heart. Keneses ha'Gedolah's Hagahos on the Tur says that people are not at all careful about eating the heart!

Poskim

Magen Avraham (OC 170:19): The Ari Zal said that things such as olives cause Amei ha'Aretz to forget. If one eats them with the known Kavanah, they help one to remember.

Eliyahu Rabah (24): This requires investigation.

Kaf ha'Chayim (Palagi, 24:43): One should not eat nuts regularly, for it causes forgetting. One who eats olives can intend 'Kel Elokim Matzpatz', which has the same Gematriya (417) as Zayis, and this counters the power of olives (to cause forgetting). Mor u'Ketzi'ah distinguishes between olives that give oil and other olives.

Kaf ha'Chayim (Sofer, 157:27): This is from the verse "Kel Elokim Hash-m." In At-Bash, Hash-m is 'Matzpatz.' Mor u'Ketzi'ah says that olives harm only when one eats them raw and fixes his meal on them, but not pickled olives that one eats for dessert. A proof that we are not concerned for pickled olives is from R. Yochanan ben Gudgedah (our text - Hachorani; R. Tzadok sent olives for him to eat - Yevamos 15b) and R. Yirmeyah (Berachos 38b), who ate a salted olive. Yefeh l'Lev says that we cannot learn from awesome Chachamim, who knew the intent to have while eating olives. It seems that Mor u'Ketzi'ah is not lenient about olives that were merely soaked in water, for this merely removes the bitterness, but they are raw like they were, and cause forgetting. It seems that one should be stringent even if they were pickled to have the above intent.

Yabi'a Omer (3 YD 8:4): People commonly fix meals on olives because there is no Isur to eat things that cause forgetting. It is only good counsel. Therefore, if one cannot find another accompaniment (to eat with bread), it is not a problem. The proof from R. Yochanan is not solid, for it was a famine year. Also, it does not say that he ate them regularly.

Salmas Chayim (1:41): One should not rely on intent (for 'Kel Elokim Matzpatz') to regularly do something explicitly against the Gemara, especially things that cause forgetting, for perhaps "Hishamer Lecha" applies. However, one may eat olives with olive oil, and then it is good to have the above intents.

Maharsha (Chidushei Agados Menachos 53b): An olive gives its oil only after we squeeze it. Similarly, Yisrael do Teshuvah only after afflictions. The Medrash says that if Yisrael do not do Teshuvah by themselves, Hash-m will arrange that a king will make decrees like Haman did, to induce them to repent. The 'end' refers to the purpose. This is only after they changed. Olives have no attribute until their end, i.e. oil. We say that just like eating olives makes one forget the Torah he knew for 70 years, olive oil restores the Torah that he knew for 70 years. This is why in the seven species of Eretz Yisrael, it does not say 'Zayis', like it says (the name of the tree or the fruit) "Gefen Te'enah..." Rather, it says "Shemen Zayis", because the olive itself is not important, rather, the oil. Similarly, the end purpose of Yisrael is not in this world, rather, in the days of Mashi'ach after they will repent.

Kaf ha'Chayim (Sofer 2:3): Eating on broken Kelim cause forgetting, because they allude to Klipos, which are the source of forgetting.

Eliyahu Rabah (170:23): Sefer ha'Gan says that one who overeats, even for a Seudas Mitzvah such as Shabbos or Yom Tov, transgresses three Lavim - Hishamer Lecha, Bal Teshaktzu (do not make yourselves detestable), for he fills his stomach like an animal, and he may not bless Birkas ha'Mazon afterwards.

Ru'ach Chayim (Hashmatos to EH p.160, cited in Shmiras ha'Guf veha'Nefesh p.96): One may not eat things that cause forgetting. Also, he endangers himself.

Minchas Yitzchak (9:8:7): People say not to eat the end of the bread, for it causes forgetting. I do not know a source for this, but also I avoid eating it. The Yerushalmi says that one must be concerned for what people are concerned for. Sefer Chasidim (261) says that whatever people consider to be dangerous, (even if there is no reason for this, it is dangerous, because the words and eyes (Ayin ha'Ra) of people damage).

Mishnah Halachos 5:101): Salami is made from all leftover meats blended together, including the heart. Poskim argue about whether something dangerous is Batel in 60, e.g. a spider that fell into water. The Darchei Moshe, Taz (YD 116:2), Maharil and Nekudas ha'Kesef say that it is not. The Pri Chodosh says that if there is 60, or even if there is not 60 and it dissolved, we are not concerned. Keneses ha'Gedolah, Pri To'ar, Shevus Yakov and Yad Efrayim are lenient (to say that there is Bitul in 60). The custom is to be lenient. People join the opinion of the Magen Avraham (179:8) that nature changed (so we may be lenient nowadays). If there were a danger, we would not rely on Bitul in 60! The Ari Zal says that eating the heart ties the Yetzer ha'Ra to a person. If so, Bitul should not apply, just like it does not apply to Chametz, due to the Yetzer ha'Ra (Chametz represents haughtiness). However, since there is no Isur to eat the heart, it seems that it is Batel in 60, and perhaps even in a majority. All the more so we may be lenient about salami, for it is a Safek whether any heart was mixed in.

Shmiras ha'Guf veha'Nefesh (p.96): Mikdeshei Hash-m says that doing an act (eating olives) to cause forgetting is no worse than passively transgressing through not reviewing. Divrei Malkiel (4:1) says that eating things that cause forgetting is not sure to cause forgetting. Also, he can eat other things that improve memory.